Method and apparatus for the control of a weight suspended from a floating vessel

ABSTRACT

The invention concerns the control of the suspended weight supported from a piston of a pneumatichydraulic system on a vessel subject to wave and tidal action and particularly a vessel employed in connection with submarine drilling operation. The particular improvement concerns the modulation of the pressure in a portion only of the system to compensate for changes in pressure in another portion of the system so as to maintain the total force on the piston substantially constant during each portion of the cycle action of the heave. This may be accomplished by supporting the load from a piston system supported by two cylindrical elements and independently modulating the pressure in one of them to compensate for variations in pressure in another of the cylinder elements.

United States Ptt Larralde et al.

[ METHOD AND APPARATUS FOR THE CONTROL OF A WEIGHT SUSPENDED FROM AFLOATING VESSEL [75] Inventors: Edward Larralde; Glen Robinson,

bothof Santa Barbara, Calif.

[73] Assignee: Vetco Offshore Industries, Inc.,

Ventura, Calif.

[22] Filed: July 13, 1973 [21] Appl. No.: 378,963

Related US. Application Data [63] Continuation-in-part of Ser. No.373,968, June 27, 1973, which is a continuation-in-part of Ser. No.274,880, July 25, 1972, Pat, No. 3,841,607.

[52] US. Cl 254/172, 175/5, 175/27,

[51] Int. Cl E2lb 15/02 [58] Field of Search 254/172, 173 R, 173 A,

[56] References Cited UNITED STATES PATENTS 3.653636 4/1972 Burrell175/5 Mar. 18, 1975 3,718,316 2/1973 Larralde 254/173 3,746,329 7/1973Galle ..267/125 Primary E.\'aminer-Robert B. Reeves AssistantEraminer-Thomas E. Kocovsky [57] ABSTRACT The invention concerns thecontrol of the suspended weight supported from a piston of apneumatichydraulic system on a vessel subject to wave and tidal actionand particularly a vessel employed in connection with submarine drillingoperation. The particular improvement concerns the modulation of thepressure in a portion only of the system to compensate for changes inpressure in another portion of the system so as to maintain the totalforce on the piston substantially constant during each portion of thecycle action of the heave. This may be accomplished by supporting theload from a piston system supported by two cylindrical elements andindependently modulating the pressure in one of them to compensate forvariations in pressure in another of the cylinder elements.

2 Claims, 3 Drawing Figures M MW PATENTEDW1 8' SHEET 1 {If 2PATENTEUHAR] 8|975 I 3,871 ,622

sum 2 OF 2 METHOD AND APPARATUS FOR THE CONTROL OF A WEIGHT SUSPENDEDFROM A FLOATING VESSEL This application is a continuation-in-part ofapplication Ser. No. 373,968 filed June 27, 1973 which in turn is acontinuation-in-part of application Ser. No. 274,880 filed July 25, 1972now U.S. Pat. No. 3,841,607.

BACKGROUND OF THE INVENTION This invention relates to improvements inmethods and apparatus for the control of a suspended weight from afloating vessel and is particularly directed to the control of forcesimposed on the drill string of a floating vessel employed in drilling,coring, running casing, reaming, cementing, testing, or other servicesin bore holes drilled in subaqueous environments where the vessel issubjected to wave or tidal action.

The particular and preferred object of this invention is to improve theoperation of such systems in which the element constituting the load issuspended from a pneumatically controlled hydraulic system. As is wellknown in the petroleum industry, the drill string, due to the greatlength above the drill collar, is a very flexible member subjected toconsiderable stretch due to its length and due to its own weight. Theweight on the bit is less than the total static weight of the drillstring because of the tension in the drill string imposed by thedrilling lines as is well understood by those skilled in this art. Thepractice during drilling is to keep the drill pipe above the drillcollar in tension. The drill collar acts as a weight-producing elementwhich exerts the load on the drill bit.

In the hydraulic-pneumatic systems of the prior art, this isaccomplished by a gas pressure in an accumulator which pressurizesliquid in a hydraulic cylinder underneath the piston which supports theweight. The'cylinder may be positioned to support a crown block orconnected to the traveling block conventional in drilling derricks.

Wave action imposes a vertical oscillatory motion on the vessel which isimposed on an hydraulic cylinder resulting in variations in the tensionin the drill pipe and therefore in variation in the load imposed uponthe drill bit, when this is employed or any other load connected to thepiston rod. In the case of the hydraulic-pneumatic systems, the pressureon the liquid underneath the piston rod is maintained by gas pressure inan accumulator; such systems are shown in the Hanes et al., U.S. Pat.No. 3,714,995 and in the Larralde et al., U.S. Pat. No. 3,718,316.

Experience with such systems has shown that a variation of about i 2 toi 5% of the suspended weight may be experienced at each wave cycle, evenwhen no drilling advance is maintained. With drill advancing duringdrilling, an additional weight variation may occur. The results of thesemotions are that the pressure in the cylinder fluctuates and the degreeof fluctuation increases as the drilling progresses.

Floating vessels operating as drilling vessels in the open sea mayexperience vertical motions, i.e., heave due to wave action ranging, forexample, from as low as 2 inches to feet or more trough to crest as, forexample, has been experienced in drilling of the North Sea. However,under ordinary conditions, the ships are on station and drilling whenthe heave is not more than about 10 to 15 feet. The wave action imposesa vertical displacement of the drilling vessel at a sinusoidalfrequency. The period of such cycles has been reported in the range of 8to 16 seconds but may be either greater or less.

The demand on the string will vary, depending on the services which theyare to provide. Thus, for landing casing or instrument survey, it isdesirable to hold the piston fixed in space at the desired level inorder that the casing or instrument is not subject to displacement.

There is also another circumstance where it may become important thatthe piston be maintained at relatively stationary position in space.Thus, when the well suddenly develops a high pressure due to theproduction of gas and it becomes necessary to close the blowoutpreventer, it becomes highly important that the drill string remainfixed in spaced and not oscillate in the closed blow-out preventer so asnot to damage it.

During drilling, however, the drill is to be advanced at a controlledrate while maintaining a controlled weight on the bit. It is desirableto maintain a desired upper load limit on the bit in order thatexcessive stresses and torque are not developed which will be so largeas to injure or even cause rupture of the drill pipe. On the other hand,it-is desirable that the load on the bit be not reduced excessively sothat the rate of advance is unreasonably reduced. Since the cost ofoperation of the drilling operation is materially effected by thedrilling rate, it is desirable that the drilling rate be maintained atas high a rate as is consistent with safety. This is established by thedriller based on his experience and the performance of the drillingoperation under consideration. The driller sets the load required forthe drilling advance to make the advance be at a satisfactory rateconsistent with safety.

In the prior art, it has been suggested that the hydraulic cylinder bemounted on the derrick to support the crown block or between thetraveling block and the hook and that a force be applied to the pistonsufficient to maintain the desired fraction of the total-load of thedrill string during drilling operation so as to hold the desired loadupon the drill bit.

STATEMENT OF OUR INVENTION In our preferred embodiment, the weight,which in our presently preferred application of our invention may be adrill string, is connected .as above to the piston of a hydrauliccylinder with the liquid under the piston in communication with apneumatic accumulator under gas pressure. It is the purpose of ourinvention to maintain a desired force on the piston in the cylinder tobe maintained during the complete wave cycle so that a predeterminedload or a predetermined load variation on the drill can be maintainednotwithstanding the amplitude or frequency or changes in the frequencyor amplitude of the wave action. Where it is desired that the drill beadvanced, ourinvention will provide a control for the advance of the bitunder a controlled load during the advance of the drill.

In our preferred embodiment, we accomplish this objective to maintain asubstantially constant force on the piston rod, exerted by separatelyapplied forces by separate pneumatic accumulators by modulating thepressure in certain thereof, whereby pressure variations in one of themarising from the tendency of the piston to move relative in space andrelative to the movement of the cylinder, due to wave action, iscompensated for by adustment of the pressure in the other accumulator.We may control force on the piston to hold the piston relatively fixedin space as when it is not desired to advance the drill string, but wemay also, while maintaining the force, so modulate the volume of thepressures in the hydraulic system cause an advance of the piston inspace as where a downward motion of the piston as in drilling is to beaccomplished. In our preferred embodiment of a pneumatic-hydraulicsystem which supports a weight and is subject to cyclic action, wemodulate the force exerted in the hydraulic-pneumatic system by imposingsaid force by independently controlled force applying means and varyingthe force imposed by at least one of them to compensate for variationsof force in another of them so as to maintain a substantially constantforce on the piston.

It is another object of our invention, in a pneumaticsystem, which issubjected to cycles of vertical displacement and which supports aweight, to maintain a substantially constant force on said weight bytrimming force changes by independently modulating a portion of saidforce so as to compensate for the said force changes and to maintain aconstant force on the system.

We accomplish .our objective by obtaining a signal responsive to theforce on the piston rod supporting said weight, comparing said signalwith a signal corresponding to a standard which is responsive to adesired force on the piston rod and thus obtain an error signal. Weindependently add or decrease a portiononly of the force exerted on thepiston rod to reduceor ideally cancel the error signal.

DETAILED STATEMENT This invention will be further understood byreference to the drawings of which:

FIG. 1 is a somewhat schematic showing of the arrangement of therelations of parts of the system of our invention.

FIG. 2 is a section of a conventional valve employed in our system.

FIG. 3 is a schematic diagram of a control system of our invention.

FIG. 1 shows the application of a control of our invention to anoperation from a floating vessel 1 acting as the drilling platform. Theconventional derrick 2 mounted on the vessel carries the split crownblock 3 from which the sheaves 5 are suspended by the drilling lines.The sheaves 5 carry a cylinder 4, vented at 7, in which is positioned apiston 8 connected to a tubular rod 9 from which is suspended theconventional hook 10 which carries a swivel 11 and the kelly 12. Thedrill pipe 14 is connected to the kelly and to the drill collar which isconnected to the bit 15. The casing 16 is composed of the conventionalmarine riser and the bore hole casing assembly together with the usualdrilling equipment.

The hollow piston rod carries a piston head 13 connected to the cylinderhead by a closed-end tube 17 which passes through a seal in the piston8. The tubular member 17 is bored at 6 to provide a communicationbetween the inlet 19 to member 17 and to annulus 18 between the tubularrod 9 and the tubular member 17. This cylinder and rod construction andits use as a weight control have been described in the aforesaidapplication, Ser. No. 274,880, which is herewith incorporated in thisspecification by this reference. The tubular member 17 is connected by apipe 19 via the solenoid valve 20, to be more fully described below, andto the reservoir 21 through valves 22, pump 24, and valve 23. Valves 22and 23 are solenoid controlled as will be described below. The reservoir21 is also in communication with the accumulator 34 through the valve27. The pump may circulate via valve 29 and also through lines 31 and 32via valve 20.

FIG. 2 illustrates a proportional metering valve 20 by which flowproportional to the magnitude and sign of the electric signal whichactivates coils 49 and is obtained as will be described below.

A torquing armature 47 is supported by a flexure tube 48 in such amanner that energizing coil 49 or coil 50 will cause torquing armature47 to move in a direction determined by the relative forces exerted bythe solenoid coils 49 and 50, moving element 51 and deflecting spring 52with reference to pin 53. The resultant movement of the valve spool 56will determine the area of the ports 61 and 61' which are uncovered anddetermine the relative flow of fluid from port 57 to ports and 62 viainterconnected ports 58, 61, and 61. When the electromagnetic forcesfrom the solenoids 49 and 50 are equal due to equal voltages applied tothe solenoid coils or when both are unenergized, the spool 56 iscentered, thus cutting off ports 57 from both ports 60 and 62. Y

Feedback shaft 53 engages feedback spring 52 which, in turn, bears onelement 51 attenuating the movement of element 51 so that movement ofelement 51 represents the summation of forces resulting from therelative elasticity of flexure tube 48, feedback spring 52, and themagnetic flux forces in coil 49 or 50, thus assuring a displacement ofvalve spool 56 in such a manner to allow flow proportional to thedifference in the electric signal to coils 49 and 50.

The valve 20 described herein is a well-known valve, and no invention isclaimed for the valve apart from its use in the combination and for thepurpose of our invention. Other valves to regulate the direction andmagnitude of flow which will function similarly in our invention may beused.

The pneumatic accumulator 30 is in communication with the cylinder 4through the valve 26. The tubular member 17 is connected to thereservoir 21 through valves 20 and 22 or via valve 20, line 32, andvalve 23. The tubular member 17 may also be connected to the accumulator30 via the by-pass line 40 and valve 41 and to the accumulator 30a whichmay be pressurized by the gas inlet through valve 30b.

The line 19 is connected to the port 62. The line 31 is connected to theport 57 and to the accumulator 34 via valve 27 and to the reservoir 21through the valve 22, pump 24 or through the regulator valve 29 to thereservoir 21. The pump during the operation of the system continuouslycirculates fluid through the by-pass valve 29. The reservoir may be atany pressure desired, e.g., it may be at ambient pressure and thepressure of the pump 24 may be set by the regulator 29. Line 32 isconnected to the port 60 and to the reservoir through valve 23 and tothe accumulator 34 through valve 27. The by-pass 40 with valve 41connects the line 31 to the accumulator 30 via the by-pass 40 with themanual valve 41. The by-pass with the manual valve 45 connects theaccumulator 34 and line 32. The valve 23 is by-passed by a manual valve43 and the valve 22 is bypassed by a manual valve 42.

The pressure in the accumulator 34 is sensed by a pressure sensor 36 togive a voltage of e responsive to the pressure in 34. A strain gauge of35 is mounted on the piston rod above the hook to give an output eproportional to the stress in the piston rod.

The stress and pressure sensors are provided with readouts which producea voltageproportional to the parameters to which they respond.

The schematic block function diagram, FIG. 3, illustrates the servocontrol of the volume of the liquid in the accumulator. All electricalelements used in the system are conventional, and their selection willbe understood by those skilled in the art to which they pertain. Theirassembly in combination with the system here described illustrates thepreferred embodiment of the control assembly of our invention.

The output voltage e of the strain gauge 35 sensor is compared with theoutput voltage e, of the pressure sensor 36 in comparator 46 to give anoutput proportional to the difference between e, and e The comparatormay be any conventional device to give a signal responsive to thedifference of two volt ages, such as a summation resistance network or adifferential amplifier or a bridge. With switches 65, 66 and 71 open(see FIG. 2), valves 22 and 23 are closed; manual valves 42, 41, 19b,45, 27 open. Liquid under pressure is available for and 300 from thepump or the pressure source 24 if used. 7 I I We prefer to employ adifferential amplifier-rectifier 46a. The inputs to the differentialamplifier are the outputs e and e and the outputs of the differentialamplifier-rectifier are applied, one to the coil 49 and the other to thecoil 50. The differential tractive effort of 49 and 50 is, therefore,proportional to the respective signals and magnitudes of e, and e Theresultant displacement of the spool 56 is, therefore proportional tothis difference. The orifices at ports 65 and 61 will depend on theaforesaid difference. The rate of addition or removal of fluid from theannulus is thus made proportional to the demand in order to establishthe desired level of forces.

During the charging of the accumulators and the cylinders and theannulus, manual switches 66 and 65 are open. The solenoids 49 and 50 arede-energized. With switch 71 open solenoids 71 and 73 are bothdeactivated, the valves 22 and 23 are closed.

With switch 66 open, the solenoids 49 and 50 are both deenergized; thespool 56 is centered; ports 60, 62, and 57 are shut off from each other;and the line 19 is closed off from line 31.

The pump 24 circulates fluid through the pressure regulator valve 29 andvia the manual valve 42, line 31, by-pass valve 41, valve 45, manualvalve 27 to the accumulator 34 and to the accumulator 30 and via themanual valves 45 and 43 back to the reservoir.

The pressure regulator 29 is set so that the pressure at 22 is above thehighest pressure attained in the annulus during operation. If thepressure source is used during operation to supply pressure to theannulus as described herein, it is established at this higher pressure.

Fluid also passes from line 31 via the by-pass line 40, valve 41 to thecylinder 4 and via the by-pass line 19a and manual valve 191; to theannulus 18 via line 19.

Pressure in the accumulators 30, 30a and 34 is adjusted by adjusting thevalve 29 and gas pressure in the accumulators and the circulationcontinued until the strain sensor 35 gives an output 2 measured at thereadout 35a (see FIG. 3) which establishes the stress required tosupport the fraction of the weight of the drill string as describedabove. The output 2, of the pressure gauge 36 at that stress is read atthe readout 360 (see FIG. 3). This pressure which corresponds to thatstress is exerted in this annulus l8 and in the cylinder 4 and in theaccumulators. The gas pressure in 34, 30, and 30a is adjusted to holdthe required pressure.

Instead of connecting the accumulator 34 to accumu lator 30 via valve34, we may use a precharged accumulator containing gas at a selectedpressure less than the minimum pressure to be attained in operation.Valve 27 may be, but need not be, used. A fixed orifice 27a, shown indotted lines, may be positioned in the line connecting the accumulator34 and line 31. The accumulator 34 is connected to the accumulator 30via valve 34. When the initial pressures are established in 30 and 30aat the level to establish the desired stress in the piston rod, the samepressure will be established in 34. The orifice is of such character andof such time constant that for the period of the heave and the pressuredifferences which are effective across the orifice the pressure in theaccumulator remain substantially constant.

The pressure in 34 and the output e is thus a reference for the forceson the piston which establishes the stress required to support thedesired load as measured by the output e; of the sensor 35.

Assume that the system is mounted on a floating vessel and moves upwardsas the heave starts. In the form shown in FIG. 1, the cylinder movesupward with respect to the piston; and the volume in'the cylinder 4under the piston 8 starts to decrease; and the volume in the annulusstarts to decrease. Liquid transfers from the cylinder 4 and from theannulus 18 to the accumulator 30, valve 26 being open, increasing thegas pressure in 30 and resulting in an increased pressure in thecylinder 4 and in the annulus. This creates an increase in the forceexerted on the piston and an increase in the stress in the piston rod.

The stress sensor output and the voltage e become greater than e theoutput voltage of the fixed reference sensor 36. The tractive effort ofsolenoid 50 exceeds that of the solenoid 49, and the spool shifts so asto place the port 57 in communication with both ports 60 and 62. Theannulus discharges through port 60.

Withe greater than 2, and with solenoids and 68 oppositely poled,solenoid 70 closes switch 68, activating the solenoid 73 to open valve23. Switch 67 remains open, and valve 22 is closed.

The pressure in the annulus 18 is vented through 19, valve 20, line 32,and valve 23 to the reservoir 21, to compensate for the increase inpressure in the cylinder 4, until the sum of the forces in the annulus18 and the cylinder 4 as reported by the sensor output e equals theoutput e,. The switch 67 opens and valve 22 closes, 23 remaining closed.The tractive effort of 50 equals that of 49.

When this occurs, the spool 56 moves to close port 57 from the ports 60and 62.

When the cylinder has reached the crest of the wave and starts todescend, the volume in the cylinder 4 increases and the volume in theannulus 18 increases. Pressure starts to fall in the cylinder and in theaccumulator 34, and the stress sensor reports a decrease in the force onthe piston rod and e falls in voltage below the output e The tractiveeffort of solenoid 49 exceeds that from the solenoid 50, and the spool56 shifts to connect the ports 62, 57, and 60. The solenoid 70 holds theswitch 68 in open position and valve 23 closed. The

switch 67 closes, energizing the solenoid 71 opening valve 22. Pressureis exerted via valve 22, through 31, port 62 to the annulus 18.

As soon as the sum of the forces exerted on the piston in the cylinder 4and annulus creates a stress so that e, e valve 22 closes as does valve20 as described above.

The system thus withdraws fluid from the annulus and accumulator 30aduring the period of the heave from the trough to the crest and addsfluid to the system during the period of the heave from the crest to thetrough in an amount and under a pressure to maintain the total force onthe piston substantially constant. The withdrawal or the addition isinterrupted when the force in the piston sensed as a stress in thepiston rod has reached a predetermined force at which the piston is tobe supported. This operation will occur even though the descent of thepiston from the drilling operation occurs. The criterion for theadditional withdrawal of liquid is the deviation of the force from apredetermined norm, which is the force desired to be maintained underthe piston under the conditions which it is sought to maintain thepiston.

Should it be desired to change the conditions to adjust the pressure,the gas pressure is adjusted to either increase or decrease the pressurein 30 and 30a as desired; and the system will automatically adjustitself to that pressure as will be evident from what has been describedabove.

As has been explained above, the motion of the piston with respect tospace is a combined motion of the piston due to the heave of thevesseland the advance of the drill during drilling.

The vessel and the cylinder are subjected to substantially sinusoidalmotions which may be out of phase with the piston which is at lesser orgreater amplitude, depending on the structure and operation conditionsof the system. This will appear from the following:

LET:

y The spacial amplitude of displacement of the cylinder at any angle ofthe sinusoidal motion.

x The spacial amplitude of displacement of the piston at said angle 6.

A The maximum displacement of the cylinder, i.el, the amplitude of theheave at the crest, which is /z of the heave.

A, The effective area of the piston, i.e., the sum of the effective areaa of the piston 8 and b the effective area of the piston 13.

V Volume of the gas in the accumulator when the system is at rest, i.e.,when 0 0.

P The pressure in the gas on the liquid at V,,.

F The force on the piston.

V, The volume of the gas in the accumulator at any angle 6 of the cycle.

P The'pressure in the accumulator when the volume is V p, The pressurein the cylinder 4 and 1 is the pressure in the annulus when p, is thepressure in the accumulator.

z The spacial advance of the bit into the earth per second.

p The period in seconds of the cyclic motion.

w The contribution to the spacial displacement of the piston due to theadvance of the drill string into the earth as in drilling per degree ofthe cycle at any angle 0 of the cycle.

The piston motion is influenced by damping considerations and moves outof phase with the motion of the cylinder. The phase angle (b depends onthe dynamics of the system.

It has been observed that the cylinder motion with respect to the spaceis sinusoidal. Being sinusoidal, the spacial displacement of thecylinder at any angle 0 of the cyclic motion may be expressed as y A sin6 and the displacement of the piston per degree of the cycle at theangle 0 of the cylinder cycle due only to the cyclic action is x A f (6)wheref( 0) is a function of the damping and other conditions of thesystem which may vary from cycle to cycle and even during any cycle andon wave condition, i.e., frequency and amplitude.

The relative displacement of the cylinder and piston, assuming noadvance of the drill occurs, is, keeping the direction signs of themotion in mind,

if the sign of y and x are the same. Thus if the sign of x is oppositeto the sign of y, then The change in volume AV of the liquid and of thegas, the total liquid volume being constant per degree of the cycle atany angle 0 is If, however, the piston descends due to the advance ofthe drill string, the total advance at any angle 6,d per degree of thecycle is At the end of each quarter cycle, the total advance of thedrill in each quarter cycle ao P Since at y =A, the net displacement ofthe piston and cylinder at the end of each quarter cycle is 90 A i am)The net displacement of the piston relative to the cylinder is A i zp/4The net change in volume of the liquid of the cylinder per degree of thecycle at any angle 0 is Since an equal change in the same sense willoccur in the gas, the change in volume of the gas AV is At the end ofeach quarter cycle, the net volume change V during the quarter cycle isFor example, assume the pressure p; and p are the same and equal to P inthe accumulator when 0 0. The pressure in the cylinder and accumulatorat any other angle will be a function of the volume change, i.e.,

where n is the polytropic gas constant which in the systems underconsideration may be taken to be 1 to 1.1 for practical purposes.

V is the volume of the gas at any angle 0. AV is the change in volume inthe accumulator at any angle 0 of any quarter of a cylce. The consequentpressure on the gas and liquid is where p, is the pressure in theaccumulator and the cylinder at V i AV.

The total force F p,a p b. In order to maintain F constant, any changein p resulting from any AV must be compensated for by an oppositeproportional change in p It is an object of our invention to maintain Fsubstantially constant and to do so we provide means to assure that theaverage pressure per square inch under the pis' ton remainssubstantially constant during the controlled operation.

We do this by adding or substracting from the pressure under the pistonin the annulus 18 an amount to compensate for the change in pressure inthe cylinder and accumulator 30.

We may use any hydraulic cylinder of design to permit the support of theload by piston areas positioned in two or more cylinders, at least oneof which contains liquid under the piston pressurized by a pneumaticaccumulator. In our preferred embodiment, described above, however, weemploy a cylinder which is a duplex hydraulic cylinder which is thesubject of the aforesaid copending application, which is herebyincorporated by this reference. In a cylinder of this character, thepiston is supported by two concentrically mounted cylindrical elementswhich move together; and the total force on the piston is the sum of theforces in each of the cylindrical elements. The elements are so arrangedthat on displacement in space one of the cylindrical elementssimultaneously increases and desreases the volume in both cylinders.

The total change in volume of the gas in the accumulator as a result ofone complete cycle is the relative displacement of the piston andcylinder resulting from the advance of the drill into the earth duringthe cycle.

In order to compensate for the variation in cylinder pressure resultingfrom the volume changes in the cylinder and in the accumulator duringheave, we modulate the pressure in the annulus to maintain asubstantially constant force on the piston.

The pressure p in the annulus is modulated upon any change in pressurein so that the force remains con stant.

As the pressure in the cylinder falls due to the transfer of liquid fromthe accumulator to the cylinder 4, the pressure in the annulus isincreased 50 compensate for the decrease in pressure in the cylinder soas to maintain a substantially constant force on the piston. When thepressure rises in the cylinder clue to transfer of liquid from thecylinder to the accumulator, the pressure in the annulus is reduced tocompensate for the increase in pressure in the cylinder. The result ofthis operation is to maintain a substantially constant force on thepiston. This is accomplished both when the piston is held atsubstantially constant position in space or is advanced with respect tospace at a constant or modified rate into the earth as in drilling.

The following example is for the purpose of illustrating the principlesof our invention and not to be understood to be any limitation thereof'The following data may be taken as representative of possible practicalconditions: Assume that a 0.45 sq. ft. and b 0.05 sq. ft., i.e., A 0.5Assume that the drill string weights 200,000 lbs. and that the desiredweight on the drill is 20,000. This requires a force on the piston of180,000. The required pressures F 0.45 x 144 .05 x 144 180,000

F/144 0.45 .05 1250 psig If at 0 0 in the first cycle 1 =p then toattain 1250 psig, pl p2 2500 psig.

Let V 100 cu. ft. and y at 6 i.e., A be taken as 5 ft. The heave is thus10 feet. Assume a period of 10 seconds, and z .02 feet per second, i.e.,a drilling rate of 72 feet per hour.

D A id 5 i [0.02 X 10/4] 5 0.05

In the first quarter cycle at the crest, i.e., 6 90, the piston hastraveled for 2.5 seconds and has descended 0.05 feet. The cylinder hasrisen 5 feet. Therefore, the decrease in volume of the liquid incylinder 4 as a result of the displacement of the piston in thecylinder:

AV 0.45 (5 0.05) 2.2725 cu. ft.

Since this is a decrease in volume of liquid in the cylinder, this is avolume which is transferred to the accumulator and the volume of the gasis decreased by this amount. During the second and third cycle when thecylinder is moving from the crest to trough and the piston is advancingduring drilling for 0.05 feet, while the cylinder is moving from crestto trough, the piston advances 0.05 feet in each quarter cycle, i.e.,during the second and third cycle will be:

+AV= 2 X .45 (5 .05) 4.455 cubic feet This volume is transferred fromthe accumulator to the cylinder 4. The volume of liquid in theaccumulator 30 has decreased and the gas volume has increased. In thelast quarter, in moving from the trough to the midpoint of the heave,the piston has descended another 0.05 feet; and the cylinder has movedup 5 feet in space; so the volume of the liquid in the cylinder hasdecreased as has the gas in the accumulator.

-AV 0.45 (5 .05 feet) 2.2725 cu. ft.

This volume is transferred to the accumulator 30, decreasing the volumeof the gas.

In going from 0 0 to the crest, in the first quarter cycle, the volumeof the liquid in the accumulator increases by 2.2725 cu. ft. at thecrest and the volume of the gas decreases by the same amount, i.e., to97.7275 cu. ft. The pressure in the accumulator and in the cylinder 4 inpounds per square inch is:

p 2500 [100/97.7275] =2558 psig In order to re-establish the averagepressure of 2500 psig and force of 180,000 pounds, the pressure in theannulus must be reduced so as to establish:

4,455 cu. ft. to 102.1825 cu. ft. and the pressure drops to 2447 psig.In order to re-establish an average pressure of 2500 psig, the annuluspressure is raised to 2980 from the 1980 psig at the crest, i.e., tore-establish a force of 180,000 lbs.

1n going from the trough to the midpoint of the rise, i.e., in the lastquarter of the first cycle, the volume of liquid in the cylinder 4 againdecreases and the liquid on transfer to the accumulator decreases thevolume of the gas by 2.2725 cu. ft. from 102.1825, resulting in a volumeof 99.91 cu. ft. and a pressure of 2502.3 psig. The pressure in theannulus must be reduced from 2981 psig to 2502.3 psig.

lt will be recognized that the process of pressure variation is acontinuous function of the cyclic movement. In order for the forces inthe piston to be maintained substantially constant, the pressure in theannulus must be adjusted continuously, as the pressure in the cylindervaries cyclically during the cyclic action.

Furthermore, the progressive changes in the volume of the gas in theaccumulator will progressively change the magnitude of the compensatingpressure in the annulus.

From the foregoing, it will be seen that the compensating pressure p atany angle 6 of any cycle is given by the general formula where 1,0 p b Fand m-l is the number of complete cycles traversed and 0 is the angle indegrees traversed in the last incomplete cycle. As will appear, thevalue of p is at a maximum at the trough when 0 270, i.e. 31r/2.

The pressure p in the accumulator 30a is at the maximum at the trough inthe first cycle since as m-l increases, p, becomes greater and thus pdiminishes. 1t drops to 0 when p becomes 2777.8 lbs. so as to by itselfestablish a force of 180,000 lbs. The volume of the gas in theaccumulator 30 is 2777.8 2500 (VJ/V1 AND cl V 2500 X 100/2777.8 90 cu.ft.

resulting from a transfer of 10 cu. ft. of liquid to the accumulator 30.This transfer occurs when the net transfer of liquid from the cylinder 4to the accumulator 30 per cycle as shown above is:

2 X 2.2725 4.455 .09 cu. ft.

The transfer of 90 cu. ft. will occur in 100 cycles, i.e., 1,000seconds. Since the piston descends at the rate of 0.02 feet/seconds; themaximum stroke available is 20 feet.

The total volume of fluid replaced to the annulus under pressure isduring the half cycle from thecrest to the trough in each cycle 2 X .05(5 .05) .495 cu. ft.

In 100 cycles 49.5 cu. ft., at an average pressure drop of 500 pounds,being the average of 1000 lbs. differer tial in the 3rd quarter of thefirst cycle to 0 psig in the 100th cycle, represents the energyexpended.

At the end of the 100th cycle, the drilling lines are again adjusted toadjust the piston in the cylinder. The pressures are again establishehdas described above to begin again the new 100-cycle phase. When, as isusual in drilling practice, weight is added to the drill string,

where additional pipe is added, the pressures reflect this added weightin order to maintain the desired weight on the bit.

1n the above description of our preferred embodiment, we have employed astress transducer as the signal to report the integrated forces on thepiston system which generates the force on the piston rod. Since this Iforce is proportional to the sum of the forces in the cylinder 4 andannulus 18, we may use any means for reporting the magnitude of thissum. For example, we may employe pressure transducers for the pressureon each cylinder, i.e., in 4 and in the annulus 18. The butput signalmay be a voltage which may be added, each multiplied by a factorproportional in one case to a and in the other, proportional to b. Themultiplied voltages may then be added in a summation network to give thesignal e which is employed as above. These expedients, multiplying andadding or subtracting voltages, will be understood by those skilled inthe art. No invention is claimed for such components except as employedin the apparatus and process of our invention.

We maintain a constant force on the drill string and a constant load onthe drill, irrespective of the variation in pressure in the cylinderresulting from cyclic action and advance of the drill string byintroducing a compensating pressure in an auxiliary cylinder and pistonwhich creates force generating pressures to maintain a substantiallyconstant force on the piston and a substantially constant stress in thepiston rod.

By monitoring the changes in the stress in the piston rod, we obtain asignal which integrates the changes during the advance of the piston indrilling. By regulating the increase or decrease of the net pressuresresponsive to change in the force on the piston, for example, by changesin stress in the piston rod, we integrate all the parameters whichaffect this force.

We do this in our preferred embodiment, as described above, by obtaininga signal which is responsive to the force on the piston during thecyclic motion of the cylinder and the advance of the piston as indrilling, if such occurs. We modulate the pressure in the annulus so asto cancel, substantially, the variation in the force signal from thatwhich is responsive to the stress desired to be maintained.

While we have described the servo control system for maintaining aconstant force on the piston by employing a hydraulic-pneumatic system,we may also use a pneumatic system. The force on each of the cylindersmay thus be a pneumatic force exerted from a source of gas pressure. Thepressures in the annulus 18 and the cylinder 4 may then be modulated byincreasing and decreasing them as described above to maintain a constantforce on the piston and constant stress on the piston rod.

We claim:

1. In an apparatus adapted to be mounted on a vessel subject to heavedue to wave action, which apparatus includes a hydraulic cylinder, afirst piston and a first piston rod in said cylinder, means to connect aload to said first piston rod, a second hydraulic cylinder and a secondpiston in said second hydraulic cylinder; said second piston operativelyconnected to said first piston, separate pneumatic accumulator systems,one each connected to each one of said cylinders for control of theforces exerted on said piston rod, the improvement which comprises meansseparately to vary the pressure in a first of said accumulator systems,to modulate the pressure in said first accumulator system to compensatefor change in pressure in a second accumulator system on imposition of aload on said first piston, whereby the forces on said piston rod aremaintained substantially constant.

2. In the apparatus of claim 1, said means to modulate said pressureincluding a source of liquid under pressure and a reservoir at apressure lower than in one of said accumulator systems, control meansfor selectively opening a first communication between the said one ofsaid accumulator systems and said source and selectively closing asecond communication between said one of said accumulator systems andsaid reservoir when said first communication is opened and control meansfor opening said second communication and closing said firstcommunication.

3. In the apparatus of claim 2, said control means comprising a signalmeans responsive to the forces imposed on the first piston rod by saidload when said apparatus is mounted on said vessel, during all portionsof the heave, means to generate a signal responsive to the forcespredetermined to be maintained on the first piston rod by said load,means to generate an error signal responsive to the differences betweensaid first and second-mentioned signals and means selectively to openand selective to close the aforesaid communications responsive to saiderror signal,

4. In the apparatus of claim 2, said communications including pipeconnections between each of the cylinders and each accumulator system, agas connection between each of the accumulator systems and a source ofgas under pressure, a pipe connected between one of said cylinders andthe accumulator system connected thereto and connected to said source,and another pipe connected between said last-named accumulator systemand said reservoir, said means to open and close said communicationsthrough said pipes including valves in each of said pipes, means to opena first valve in one of said first-mentioned pipes and means to close asecond valve in the other of said pipes when the first valve is openedand means to open said second valve when said first valve is closed.

5. In the apparatus of claim 4, control means to open the first valve insaid pipe connected to said first accumulator system and said source andto close a second valve in the pipe connecting said first accumulatorsystem to said reservoir system, when The force imposed on said pistonrod by said lead when said apparatus is mounted on a vessel, issubstantially below a predetermined value and control means to open thesecond valve and close the first valve when the force exerted on saidpiston rod is substantially above said predetermined value.

6. In the apparatus of claim 5, said control means comprising means togenerate a signal responsive to the forces imposed on the first pistonrod in said first cylinder, during all portions of the heave, means togenerate a signal responsive to the forces predetermined to bemaintained on the first piston rod, means to generate an error signalresponsive to the differences between said first and second-mentionedsignals and means to selectively open and selectively close theaforesaid valves responsive to said error signal.

7. In combination with a drilling vessel, a derrick, drilling linessuspended from said derrick, a hydraulic cylinder and piston-pneumaticaccumulator system for control of the forces in said piston rod by aload connected to the piston rod, the improvement which comprises aplurality of accumulators, a plurality of pistons connected to the samepiston rod, means to establish pressures separately on said pistons,said means including a cylinder element for each piston, one of saidpneumatic accumulators connected to each cylinder, a source of liquidunder pressure, a reservoir, a source of gas pressure for eachaccumulator, a first pipe means to connect to a first one of saidaccumulators to said source, and a second pipe means to connect saidfirst one of said accumulators to said reservoir, and means openingcommunication through said pipe means between said source and said firstone of said accumulators and closing said communication through saidsecond pipe means, during that portion only of the heave when the volumeof the liquid in a second one of said accumulators tends to decrease andmeans to open communication through said second pipe means and closingcommunication through said first pipe means during that portion only ofthe heave when the volume of liquid in the said second one of saidaccumulators tends to increase.

8. In the apparatus of claim 7, said means to open and closecommunication through each of said pipe means, valves in each of saidpipe means and means responsive to said error signal to close and openthe valves in said pipe means.

9. In the apparatus of claim 7', said control means comprising a meansto generate a signal responsive to the forces imposed on the piston rodduring all portions of the heave, means to generate a signal responsiveto a predetermined stress to be maintained in the piston rod, means togenerate an error signal responsive to the differences between saidfirst and second-mentioned signals, and means to selectively open andselectively close the aforesaid communications responsive to said errorsignal.

10. In the apparatus of claim 9, said control means including meansselectively to open a first valve in said first pipe means and to closea second valve in said second pipe means when the said force issubstantially below a predetermined value and means to close said firstvalveand open the second valve when the force exerted on said piston issubstantially above said predetermined value.

11. A method of controlling the spacial displacement of a piston rodconnected to a piston in a cylinder which is mounted for verticaldisplacement on a vessel subject to wave action, and in which saidpiston is loaded by a weight connected to said piston, which load isopposed by liquid under pressure in said cylinder under the said piston,communicated from separate bodies of liquid under gas pressure containedin separate pneumatic accumulators, the steps of withdrawing liquid froma first one of said cylinders to a first one of said plurality ofpneumatic accumulators and from said first one of said accumulators todecrease the pressure in said first one of said accumulators, when thevolume of liquid in a second one of said plurality of pneumaticaccumulators is increasing and adding liquid to said first one of saidpneumatic accumulators to increase the pressure in said first one ofsaid pneumatic accumulators whereby the force on said piston rod ismaintained at a substantially constant value.

12. The method of claim 11, the step of advancing the piston spaciallydownward during each portion of the heave cycle and withdrawing fromsaid firstmentioned said first one of said accumulators to an extraneouspoint a quantity of liquid responsive to the said spacial advance.

13. In the process of claim 12, said withdrawal occurringonly duringthat portion of the heave when the volume under the pistons in thecylinders is decreasing.

14. In the process of claim 13, introducing into the first one of saidaccumulators a quantity of liquid less than the quantity withdrawn, saidintroduction occurring only during that portion of the heave when saidvolume in said cylinders is increasing.

15. In an apparatus adapted to be mounted on a vessel subject to heavedue to wave action, which apparatus includes a first cylinder, a firstpiston, and a first piston rod in said first cylinder, a second cylinderand a second piston in said second cylinder, said second pis ton in saidsecond cylinder operatively connected to said first piston rod, a fluidpressure source connected to one of said cylinders for control of theforces exerted on said piston rod, which comprises means separately tovary the pressure in one of said cylinders, to modulate the pressure inone of the cylinders to compensate for change in pressure in another ofsaid cylinders, whereby the forces on said piston rod are maintainedsubstantially constant.

16. In the apparatus of claim 15,.a control means comprising a signalmeans responsive to the forces imposed on the first piston rod, duringall portions of the heave, means to generate a signal responsive to theforces predetermined to be maintained on the first piston rod, means togenerate an error signal responsive to the differences between saidfirst and secondmentioned signals and means to adjust the pressure inone of said cylinders responsive to said error signal.

17. In combination with a drilling vessel, a derrick, drilling linessuspended from said derrick, a cylinder, and a gas pressure system forcontrol of the forces in said piston rod, a plurality of pistonsconnected to the same piston rod, means to establish separate pressureson said pistons, said means including a cylinder element for eachpiston, a source of pressure connected to each cylinder, means toincrease the pressure in one of said cylinders during that portion onlyof the heave when the volume under the piston on the other of saidcylinders tends to decrease and means to decrease the pressure in saidone cylinder during that portion only of the heave when the volume underthe piston in the other cylinder tends to increase.

18. In the apparatus of claim 17, said means comprising a means togenerate a signal responsive to the forces imposed on the piston rodduring all portions of the heave, means to generate a signal responsiveto a predetermined stress to be maintained in the piston rod, means togenerate an error signal responsive to the differences between saidfirst and second-mentioned signals, and means to selectively increaseand decrease said pressure responsive to said error signal.

19. A method of controlling the spacial displacement of a piston rodconnected to a pair of pistons, one in each of a pair of cylinders whichare mounted for vertical displacement on a vessel subject to waveaction, and in which said piston rod is loaded by a weight connected tosaid piston rod, which load is opposed by fluid pressure in each ofsaid-cylinders under the said pistons, the steps of reducing the fluidpressure in one of said cylinders when the pressure in the other of saidcylinders is increasing and increasing the pressure in said one of saidcylinders when the pressure in the other of said cylinders is decreasingwhereby the force on said piston rod is maintained at a substantiallyconstant value.

20. The method of claim 19, the step of advancing the piston rodspacially downward during each portion of the heave cycle.

, ECHO Patent No. 3,871,622 Dated Mar. 18, 1975 Inventor) EDWARDLARRALDE and GLEN ROBINSON It is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

In the Abstract, lines 2 and. 3, change "pneumatichy-draulic" topneumatic-hydraulic Column 3, lines 17 and 18, after "pneumatic" inserthydraulic Column 3, lines 50-51, insert piston before "rod.

Column 5, line 35, change "signals" to signs Column 5, line 45, after"open" insert Column 6, line 17, change "34" to 34' Column 6, line 64,change "Pressure" to pressure and insert Gas before "pressure."

Column 8, line 36, change the formula from i 66 V A (yix) to i v A (yiColumn 9, line 13, change the formula P (v from p .p (V (V V) to p 2 1 oo 1.. A 1 (V1 1 A Wn Column 9, line 27, correct the spelling of"subtracting."

Column 10, line 14, correct the spelling of "weighs.

Column 10, line 22, change "pl p2" to p p Patent NO. Dated Mar. 18,

EDWARD LARRALDE and GLEN ROBINSON Inventor(s) It is certified that errorappears in the above-identified patent that said Letters Patent arehereby corrected as shown below:

Column 11, line 44, change the formula to read:

I 1 2 144b Y o P i aAsinG z??? j Column 11, line 60, delete "cl" after"AND."

Column 12 line 1, change to Column 12, line 14, correct the spelling of"established,"

Column 12, line 27, correct the spelling of "employ."

Column 13, line 42, change "selective" to selectively Column 15, lines14-19 claim 12 should read as follows:

12. The method of claim ll, the step of advancing the piston spaciallydownward during each portion of the heave cycle and withdrawing fromsaid firstmentioned accumulator to an extraneous point a quantity ofliquid responsive to the said spacial advance.

Signed and ficalcd this twenty-eight Day 0f October 1975 [SEAL] Arrest"RUTH C. MASON C. MARSHALL DANN Arresting Officer Commissioner of Patentsand Trademarks

1. In an apparatus adapted to be mounted on a vessel subject to heavedue to wave action, which apparatus includes a hydraulic cylinder, afirst piston and a first piston rod in said cylinder, means to connect aload to said first piston rod, a second hydraulic cylinder and a secondpiston in said second hydraulic cylinder; said second piston operativelyconnected to said first piston, separate pneumatic accumulator systems,one each connected to each one of said cylinders for control of theforces exerted on said piston rod, the improvement which comprises meansseparately to vary the pressure in a first of said accumulator systems,to modulate the pressure in said first accumulator system to compensatefor change in pressure in a second accumulator system on imposition of aload on said first piston, whereby the forces on said piston rod aremaintained substantially constant.
 2. In the apparatus of claim 1, saidmeans to modulate said pressure including a source of liquid underpressure and a reservoir at a pressure lower than in one of saidaccumulator systems, control means for selectively opening a firstcommunication between the said one of said accumulator systems and saidsource and selectively closing a second communication between said oneof said accumulator systems and said reservoir when said firStcommunication is opened and control means for opening said secondcommunication and closing said first communication.
 3. In the apparatusof claim 2, said control means comprising a signal means responsive tothe forces imposed on the first piston rod by said load when saidapparatus is mounted on said vessel, during all portions of the heave,means to generate a signal responsive to the forces predetermined to bemaintained on the first piston rod by said load, means to generate anerror signal responsive to the differences between said first andsecond-mentioned signals and means selectively to open and selective toclose the aforesaid communications responsive to said error signal. 4.In the apparatus of claim 2, said communications including pipeconnections between each of the cylinders and each accumulator system, agas connection between each of the accumulator systems and a source ofgas under pressure, a pipe connected between one of said cylinders andthe accumulator system connected thereto and connected to said source,and another pipe connected between said last-named accumulator systemand said reservoir, said means to open and close said communicationsthrough said pipes including valves in each of said pipes, means to opena first valve in one of said first-mentioned pipes and means to close asecond valve in the other of said pipes when the first valve is openedand means to open said second valve when said first valve is closed. 5.In the apparatus of claim 4, control means to open the first valve insaid pipe connected to said first accumulator system and said source andto close a second valve in the pipe connecting said first accumulatorsystem to said reservoir system, when the force imposed on said pistonrod by said load when said apparatus is mounted on a vessel, issubstantially below a predetermined value and control means to open thesecond valve and close the first valve when the force exerted on saidpiston rod is substantially above said predetermined value.
 6. In theapparatus of claim 5, said control means comprising means to generate asignal responsive to the forces imposed on the first piston rod in saidfirst cylinder, during all portions of the heave, means to generate asignal responsive to the forces predetermined to be maintained on thefirst piston rod, means to generate an error signal responsive to thedifferences between said first and second-mentioned signals and means toselectively open and selectively close the aforesaid valves responsiveto said error signal.
 7. In combination with a drilling vessel, aderrick, drilling lines suspended from said derrick, a hydrauliccylinder and piston-pneumatic accumulator system for control of theforces in said piston rod by a load connected to the piston rod, theimprovement which comprises a plurality of accumulators, a plurality ofpistons connected to the same piston rod, means to establish pressuresseparately on said pistons, said means including a cylinder element foreach piston, one of said pneumatic accumulators connected to eachcylinder, a source of liquid under pressure, a reservoir, a source ofgas pressure for each accumulator, a first pipe means to connect to afirst one of said accumulators to said source, and a second pipe meansto connect said first one of said accumulators to said reservoir, andmeans opening communication through said pipe means between said sourceand said first one of said accumulators and closing said communicationthrough said second pipe means, during that portion only of the heavewhen the volume of the liquid in a second one of said accumulators tendsto decrease and means to open communication through said second pipemeans and closing communication through said first pipe means duringthat portion only of the heave when the volume of liquid in the saidsecond one of said accumulators tends to increase.
 8. In the apparatusof claim 7, said means to open and close communication through each ofsaid pipe means, valves In each of said pipe means and means responsiveto said error signal to close and open the valves in said pipe means. 9.In the apparatus of claim 7, said control means comprising a means togenerate a signal responsive to the forces imposed on the piston rodduring all portions of the heave, means to generate a signal responsiveto a predetermined stress to be maintained in the piston rod, means togenerate an error signal responsive to the differences between saidfirst and second-mentioned signals, and means to selectively open andselectively close the aforesaid communications responsive to said errorsignal.
 10. In the apparatus of claim 9, said control means includingmeans selectively to open a first valve in said first pipe means and toclose a second valve in said second pipe means when the said force issubstantially below a predetermined value and means to close said firstvalve and open the second valve when the force exerted on said piston issubstantially above said predetermined value.
 11. A method ofcontrolling the spacial displacement of a piston rod connected to apiston in a cylinder which is mounted for vertical displacement on avessel subject to wave action, and in which said piston is loaded by aweight connected to said piston, which load is opposed by liquid underpressure in said cylinder under the said piston, communicated fromseparate bodies of liquid under gas pressure contained in separatepneumatic accumulators, the steps of withdrawing liquid from a first oneof said cylinders to a first one of said plurality of pneumaticaccumulators and from said first one of said accumulators to decreasethe pressure in said first one of said accumulators, when the volume ofliquid in a second one of said plurality of pneumatic accumulators isincreasing and adding liquid to said first one of said pneumaticaccumulators to increase the pressure in said first one of saidpneumatic accumulators whereby the force on said piston rod ismaintained at a substantially constant value.
 12. The method of claim11, the step of advancing the piston spacially downward during eachportion of the heave cycle and withdrawing from said first-mentionedsaid first one of said accumulators to an extraneous point a quantity ofliquid responsive to the said spacial advance.
 13. In the process ofclaim 12, said withdrawal occurring only during that portion of theheave when the volume under the pistons in the cylinders is decreasing.14. In the process of claim 13, introducing into the first one of saidaccumulators a quantity of liquid less than the quantity withdrawn, saidintroduction occurring only during that portion of the heave when saidvolume in said cylinders is increasing.
 15. In an apparatus adapted tobe mounted on a vessel subject to heave due to wave action, whichapparatus includes a first cylinder, a first piston, and a first pistonrod in said first cylinder, a second cylinder and a second piston insaid second cylinder, said second piston in said second cylinderoperatively connected to said first piston rod, a fluid pressure sourceconnected to one of said cylinders for control of the forces exerted onsaid piston rod, which comprises means separately to vary the pressurein one of said cylinders, to modulate the pressure in one of thecylinders to compensate for change in pressure in another of saidcylinders, whereby the forces on said piston rod are maintainedsubstantially constant.
 16. In the apparatus of claim 15, a controlmeans comprising a signal means responsive to the forces imposed on thefirst piston rod, during all portions of the heave, means to generate asignal responsive to the forces predetermined to be maintained on thefirst piston rod, means to generate an error signal responsive to thedifferences between said first and second-mentioned signals and means toadjust the pressure in one of said cylinders responsive to said errorsignal.
 17. In combination with a drilling vessel, a derrick, drillinglines suspenDed from said derrick, a cylinder, and a gas pressure systemfor control of the forces in said piston rod, a plurality of pistonsconnected to the same piston rod, means to establish separate pressureson said pistons, said means including a cylinder element for eachpiston, a source of pressure connected to each cylinder, means toincrease the pressure in one of said cylinders during that portion onlyof the heave when the volume under the piston on the other of saidcylinders tends to decrease and means to decrease the pressure in saidone cylinder during that portion only of the heave when the volume underthe piston in the other cylinder tends to increase.
 18. In the apparatusof claim 17, said means comprising a means to generate a signalresponsive to the forces imposed on the piston rod during all portionsof the heave, means to generate a signal responsive to a predeterminedstress to be maintained in the piston rod, means to generate an errorsignal responsive to the differences between said first andsecond-mentioned signals, and means to selectively increase and decreasesaid pressure responsive to said error signal.
 19. A method ofcontrolling the spacial displacement of a piston rod connected to a pairof pistons, one in each of a pair of cylinders which are mounted forvertical displacement on a vessel subject to wave action, and in whichsaid piston rod is loaded by a weight connected to said piston rod,which load is opposed by fluid pressure in each of said cylinders underthe said pistons, the steps of reducing the fluid pressure in one ofsaid cylinders when the pressure in the other of said cylinders isincreasing and increasing the pressure in said one of said cylinderswhen the pressure in the other of said cylinders is decreasing wherebythe force on said piston rod is maintained at a substantially constantvalue.
 20. The method of claim 19, the step of advancing the piston rodspacially downward during each portion of the heave cycle.